Inter-rib sag reduction

[timi]

Hello!

I am looking at covering an aluminum tube rib wing, and I am trying to minimize the inter-rib sag as much as possible.
I am currently very uniformed about the fabric covering process.

Is there any way to reduce the sag between the ribs after the fabric has been heat shrunk?
Does the ammount of sag depend on the type or weight of the fabric?
Or would the only way to reduce the sag be to cover the leading edge with a rigid covering such as sheet aluminum?

Thank you

[Dana]

If you don't have a rigid sheet around the leading edge, there will be some sag. More spanwise tension can reduce the sag to some extent. But is it really an issue? Many aircraft designs have unsheeted leading edges, with resulting sag between ribs, and fly just fine.

[cwilliamrose]

One well known builder/restorer says shrinking first with span-wise passes of the iron helps to minimize the sag a lot. I have not had a chance to try this myself but I have seen his work.

[timi]

Hi thanks for the replies.
I was concerned with sagging because the wing we are trying to build will be one with significant sweep, (28 deg)
and I was also concerned the sag would effect the characteristics in a way that the CFD did not predict.

I think I may end up attaching a rigid aluminum sheet to the wing's leading edge, would you all recommend
blind riveting the sheet aluminum to the aluminum tube ribs?
Thanks again.

[rwanttaja]

Hi thanks for the replies.
I was concerned with sagging because the wing we are trying to build will be one with significant sweep, (28 deg)
and I was also concerned the sag would effect the characteristics in a way that the CFD did not predict.

I think I may end up attaching a rigid aluminum sheet to the wing's leading edge, would you all recommend
blind riveting the sheet aluminum to the aluminum tube ribs?
Thanks again.

Shouldn't need many rivets. I'll leave it to guys like Frank and Joe to give a more-knowledgeable assessment, but it sounds good to me.

Standard practice on the Fly Baby is to install ~0.025" gutter flashing across the front of all the wing, horizontal stabilizer, and fin. For more of an antique look, Kurt Gruber left it off the vertical tail:
http://www.bowersflybaby.com/pix/grubert_rudder.jpg
You can see this plane with the stock metal under the fabric Note that the fin ribs are almost invisible in comparison:
http://www.bowersflybaby.com/pix/n4629T.jpg

Ron Wanttaja

[Sam Buchanan]

Hi thanks for the replies.
I was concerned with sagging because the wing we are trying to build will be one with significant sweep, (28 deg)
and I was also concerned the sag would effect the characteristics in a way that the CFD did not predict.

I think I may end up attaching a rigid aluminum sheet to the wing's leading edge, would you all recommend
blind riveting the sheet aluminum to the aluminum tube ribs?
Thanks again.

Your wing sounds quite similar in construction with the tubular wings on the Airdrome WWI replica aircraft. Here is the leading edge skin on one of the wings for my Fokker D.VII kinda replica:

7585

The leading edge is attached to the spar and ribs with pulled rivets. I also added aluminum angle to the trailing edge of the sheeting to prevent the fabric from deforming the sheeting:

7586

Hope this helps.

[Bill Berson]

Extra nose ribs to reduce the fabric span between main ribs are often used. The majority of lifting force and pull on the fabric is near the leading edge.

[George Sychrovsky]

You can reduce the sagging significantly by making the fabric a lot tighter in span-wise direction and loser in chord direction
The way to do it you first attach the fabric at the leading and trailing edges but you create about 1.5 inch slack of extra fabric in the chord dimension.
Then shrink the fabric at the root and the tip enough to remove this slack and attach the fabric at the root rib and tip keeping it as tight as possible span-wise.
Now shrink the rest.
At least that's what I did it on my two airplanes where the sagging even tends to contact the internal wing parts and It works.

[martymayes]

You can see this plane

Ron have you ever considered putting together a CD of Fly Baby pictures? No words, just pictures. That's the kind of books I like, just pictures.

[rwanttaja]

Ron have you ever considered putting together a CD of Fly Baby pictures? No words, just pictures. That's the kind of books I like, just pictures.

Nahhhh, too much work. But most of the photos are online.

http://www.bowersflybaby.com/pix/index1.htm (and click the "next" buttons)

http://www.bowersflybaby.com/stories/altered.HTM

http://www.bowersflybaby.com/goofy_pix.html

http://www.bowersflybaby.com/pix/panels.html

http://www.bowersflybaby.com/pix/portraits.html

http://www.bowersflybaby.com/pix/hans_silhouette.jpg

Ron Wanttaja

[timi]

The leading edge is attached to the spar and ribs with pulled rivets. I also added aluminum angle to the trailing edge of the sheeting to prevent the fabric from deforming the sheeting:



Wow that looks great thank you, the angle aluminum reminds me of like.. a stiffened skin structure or something! Did you use rivets to attach the angle aluminum to the skin also?


Shouldn't need many rivets. I'll leave it to guys like Frank and Joe to give a more-knowledgeable assessment, but it sounds good to me.

Standard practice on the Fly Baby is to install ~0.025" gutter flashing across the front of all the wing, horizontal stabilizer, and fin. For more of an antique look, Kurt Gruber left it off the vertical tail:




Oh perfect that doesn't sound too difficult, thank you, and may I ask, is the aluminum sheeting hard to bend by hand? The wing we are trying to make has a different shape at every rib, we are actually trying to make a crude approximation of Albion Bower's Bell Shaped Distribution Wing, if we are able to.

Oh, I have a picture of the wing shape we are aiming for, it isn't finished yet but should look something like this:
https://i.imgur.com/nDfH03L.png

Thanks for all the help, wow there are a lot of experienced folks here.. I will definitely have a lot more questions once we have started our build if you all wouldn't mind entertaining them!

Thanks very much

[Sam Buchanan]

Wow....that will be a very complex wing to build.......forget the photos I posted, you are going to be in much taller weeds.......

[George Sychrovsky]

Yeah , Looks like we got suckered into believing he had the wing to cover.

[timi]

Wow....that will be a very complex wing to build.......forget the photos I posted, you are going to be in much taller weeds.......

I'm definitely concerned that it will be beyond our ability to manufacture, each wing should have a total of 20 sections. I wasn't able to achieve a nice bell-shaped lifting distribution without some pretty wild variations in twist.. hopefully we will be able to come close and I'm not in too far over my head.

[Dana]

What kind of aircraft are you trying to build? That sort of shape is more suited to composites than aluminum tube and fabric construction. And it will likely have some rather unfriendly stall characteristics.

[timi]

What kind of aircraft are you trying to build? That sort of shape is more suited to composites than aluminum tube and fabric construction. And it will likely have some rather unfriendly stall characteristics.

Just a small ultralight with a bell spanload wing. It's my understanding that a bell spanload wing is supposed to have a nice stall beginning around 20 percent of the span from the root, and even with those skinny old tips I think it shouldn't be tipstalling weirdly. I also hope to give the structure a pretty big safety factor.

I definitely understand that composite material would be much better for this kind of structure, but I have help on the project from someone who has a lot of metal fabrication experience and tools, relative to me at least. I hope I'm not too off base.

[cwilliamrose]

If you use the angles to stiffen the aft edge of the leading edge skin as Sam shows in post #6 think about making the skins a little oversized so you have maybe 3/4" extra material along the aft edges. That extra material could then be bent 90° to create the angle without needing any fasteners or suffering the extra weight from additional parts. Once the angles are bent you would only need to provide relief for the ribs. Or you could cut the reliefs first and bend the angles one at a time instead of all at once which would eliminate the need for a large brake.

[Sam Buchanan]

If you use the angles to stiffen the aft edge of the leading edge skin as Sam shows in post #6 think about making the skins a little oversized so you have maybe 3/4" extra material along the aft edges. That extra material could then be bent 90° to create the angle without needing any fasteners or suffering the extra weight from additional parts. Once the angles are bent you would only need to provide relief for the ribs. Or you could cut the reliefs first and bend the angles one at a time instead of all at once which would eliminate the need for a large brake.

You just explained why I used angles instead of modifying the leading edge. :)

[cwilliamrose]

His wing has two sweep angles so maybe the individual parts won't be real long. If they are doing the bends in small sections is workable with some very simple tooling.

But I do understand your choice. :)

[Bill Berson]

If that's a cantilever wing it will need more or thicker stressed skin than a strut braced wing.
A D-cell skin must be riveted to the spar to make a complete D tube.

[pylon500]

I think we're going to need a lot more info on your structural concepts to give any real advice.
I don't think anyone would try to build a wing of that shape and have it externally braced, so I'm assuming you're going the Bede route and having a large alloy tube main spar.
Although this seemed like a good idea from Bede at the time, using a large diameter alloy tube with fibreglass skins (BD-4 etc) worked until they decided to skin the wing with aluminium.
The problem now was that the tube spar was capable of flexing more than the skin sheeting, and caused buckling of the skins.
Putting thicker skins tended to negate the reason for the tube spar, as the skins now did most of the work, and concentrate the loads at the root of the spar.
Watch the wing in this vid;
<https://www.youtube.com/watch?v=bu9jpREQ56A>
Better to design a propper 'D' cell wing.

[Frank Giger]

You said the magic word - ultralight - so I'll jump in.

Want to reduce the "sag" between ribs? Don't shrink the fabric to full tightening. You probably don't want to do it anyway, as light structures are easy to warp when shrinking fabric!

Next up, my advice is not to worry about it too much. Your ultralight should have a pretty fat airfoil, so a little "sag" isn't going to hurt a thing.

That is if you're using ceconite in the first place. Look at the powered hang glider craft - they use sail cloth sewed as an envelope instead.